Optical harmonic generating medium and modulating device

ABSTRACT

This disclosure is directed to a device which generates an enhanced third harmonic light wave of the neodymium laser by the introduction of anomalous dispersion into a liquid medium to produce phase matching of the interacting waves, and applications thereof which depend upon the state of polarization of the incident laser radiation. The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

states mm Inventors Appl. No.

Filed Patented Assignee Paul P. Bey

Oxon Hill;

John F. Giuliani, Kensington; Herbert Rabin, Bethesda, all of, Md.

Sept. 25, 1968 Aug. 17, 1971 The United States of America as representedby the Secretary of the Navy OPTICAL HARMONIC GENERATHNG MEDIUM ANDMODULATING DEVICE 4 Claims, 3 Drawing Figs.

U.S. Cl 307/883, 32 1 /69 int. Cl HOZm 5/00 Field of Search 307/883;330/5;32l/69 [56] References Cited UNITED STATES PATENTS 3,364,433l/i968 Freund et al 330/5 3,389,269 6/1968 Giordmaine et a]. 307/8833,390,278 6/1968 Giordmaine et a1. 307/883 Primary Examiner-Roy LakeAssistant Examiner-Darwin R. Hostetter Att0meys-R. S. Sciascia and A. L.Branning ABSTRACT: This disclosure is directed to a device whichgenerates an enhanced third harmonic light wave of the neodymium laserby the introduction of anomalous dispersion into a liquid medium toproduce phase matching of the interacting waves, and applicationsthereof which depend upon the state of polarization of the incidentlaser radiation.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

FIG. 3

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"n PAUL P. 85 Y JOHN F. GIULIANI HERBERT RAB/N m; 2' $14.. AGENT WeATTORNEY INVENTORS OPTICAL HARMONIC GENERATING MEDIUM AND MODULATINGDEVICE This invention is directed to a device for generating a thirdharmonic light wave and more particularly to a device and a nonlinearmedium for generating a phase-matched third harmonic light wave of theneodymium laser.

When high intensity laser radiation passes through a medium, light wavesare produced with frequencies which are multiples of the original laserfrequency. I-Ieretofore, enhanced second and third harmonic light, hasbeen generated by phasematching the laser and its associated harmonicwaves in birefringent crystals. The phase-matching condition requiresthat the crystal axes be accurately oriented with respect to thedirection of the incident laser beam. This insures that efficientharmonic radiation can be generated by the fundamental laser radiation.

In addition to the high cost of cutting, polishing and mounting crystalsfor harmonic type generation, there are a number of properties ofcrystals which limits the maximum harmonic power that can be generated.These include: (1) the divergence between the fundamental and harmonicwaves in the crystal, (2) dielectric breakdown in the crystal producedby high power laser irradiation, and (3) divergence of a focused laserbeam in the crystal.

The device of this invention provides a means for generating the thirdharmonic (353 millimicrons) of the neodymium laser line (1.06 microns)by the employment of anomalous dispersion in a nonlinear liquid mediumfor phase-matching the laser and its third harmonic wave. The normallyunmatched dispersive liquid employed in this device is the solventhexafluoroacetone sesquihydrate, (CF9 CO1.5 H to which has been added adye fuchsin to produce an anomalous dispersion; phase-matching isachieved at a dye concentration of approximately 45 grams of dye perliter of solvent. This specific solvent in a transparent containerconstitutes an element in the present system for generating the thirdharmonic of the neodymium laser line.

An alternate liquid medium which may be similarly employed ishexafluoroisopropanol, (CF CI-IOH, with a lesser amount of the dyefuchsin to produce phase-matching, at approximately 37.5 gm. per literliquid.

The invention may be employed to generate a third harmonic wave of alaser source by propagating the laser radiation through an isotropic oranisotropic nonlinear medium. When an anisotropic medium is used thedirection of the laser radiation must be propagated along a threefoldaxis of the anisotropic medium. In case of a liquid which is isotropic,there is no preferred axis.

A more complete discussion of this device, the fluids used therein, andits use may be found in the following articles: (1) Generation Of APhase-Matched Optical Third Harmonic By Introduction Of AnomalousDispersion Into A Liquid Medium Physical Review Letters, Vol. 19, No.15, 9 Oct. 1967, pages 819-82l. (2) Linear And Circular Polarized LaserRadiation In Optical Third Harmonic Generation, published in PhysicsLetters, Vol. 26A, No. 3, 1 Jan. 1968, pages 128- 129; and (3)Phase-Matched Optical Harmonic Generation In Liquid Media EmployingAnomalous Dispersion. IEEE Journal of Quantum Electronics number 1 1,Vol. 054, 932- 939, Nov. 1968.

It is therefore an object of this invention to provide a simple andinexpensive means for producing phase-matched third harmonic wave from afundamental neodymium laser wave.

Another object is to provide a liquid for generating a third harmonicwave from a fundamental linear polarized wave which can withstand veryhigh laser intensity.

Yet another object is to provide a liquid for the production of a thirdharmonic signal independent of the direction of a laser beam inpropagating through the device.

Other and more specific objects of this invention will become apparentupon a careful consideration of the following detailed description whentaken together with the accompanying drawings in which:

FIG. 1 illustrates a third harmonic generating cell which may beemployed in accordance with the teachings of this invention;

FIG. 2 illustrates a third harmonic generating cell in an optical systemin which a neodymium laser wave is linearly polarized; and

FIG. 3 illustrates a third harmonic generating cell in an optical systemin which a neodymium laser wave is circularly polarized.

Now referring to the drawings, there is shown by illustration in FIG. 1,a third harmonic generating cell which is used in the systems shown byillustration in FIGS. 2 and 3. This cell may be a commercially availableshort path length, type DX, liquid cell made by the American InstrumentCompany. The cell is made of coaxially arranged tubular members 6 and 7with a spacing between the tubular members and parallel end plates 8 and9, which enclose one of the ends of each of the tubular members. Thefree ends of the tubular members are secured to each other to seal offthe ends of the tubular members. The container is provided with a filleropening 10 and a stopper 11 to seal the fluid within the container. Thelinear spacing between the tubular members forms a reservoir and thespacing between the parallel end plates provide a window through which alaser beam passes.

A cell formed in such an arrangement allows heat generated by thepassage of a laser beam through the fluid between the parallel endplates to be carried to the reservoir. The reservoir-container 5includes therein a harmonic generating solution 12 containing the dyefuchsin dissolved in either of two solvents, hexafluoroacetonesequihydrate (CF CO.l.5 H 0 or hexafluoroisopropanol (CF CHOI-I. The dyeconcentrations are approximately 45 grams per liter of solvent and 37.5grams per liter of solvent, respectively, and the path length betweenthe parallel end plates is about 0.05 centimeters. The

third harmonic generating solution is formed by combining the dye withthe solvent and then pouring the solution into the container through theopening 10 until the container is filled. The stopper 11 is then placedin the filler opening to prevent fluid loss. Thus, the solution iscontained between the walls of the coaxially aligned cylindrical membersand the end members that form the window through which the laser beampasses.

FIG. 2 illustrates the third harmonic generating cell 5 in alaser-optical system. As shown, a fundamental linearly polarized lightsource generated by a neodymium laser 13 is directed onto a beamsplitter 14 which reflects a portion of the light onto a photodiode 15and and permits the remainder of the light to pass. The light thatpasses through the beam splitter 14 is directed onto the third harmonicgenerator cell 5 where the optical interactions of the light inpropagating through the medium 12 produces a third harmonic wave whichemerges from the cell. The third harmonic wave then passes through afilter pack 16 and is detected by a photomultiplier tube 17. The outputsignal produced by the photomultiplier tube is applied to theY-amplifier of an oscilloscope 18 which produces a signal as shown. Thesignal shown, illustrates a third harmonic pulse (inverted). Thephotodiode 15 produces an output signal representative of the lightpulse as generated by the laser which is applied to an oscilloscope 21producing a pulse as shown. The oscilloscope 21 is connectedelectrically with the scope 18 to trigger the sweep of scope 18 upon thereceipt of a signal from the diode 15.

FIG. 3 illustrates an arrangement similar to that of FIG. 2, however,the system includes a Fresnel rhomb 22 in the light path before thethird harmonic generating cell 5. The Fresnel rhomb converts thefundamental linearly polarized light to circularly polarized light asrepresented by the circular arrow. Other optical elements or means maybe similarly employed to obtain circularly polarized light. Thecircularly polarized light is then incident on the cell 5 and there isno third harmonic output from the cell as shown by the scope 18. Again,the scope 21 shows the signal due to the laser pulse incident onphotodiode 15. Therefore, FIGS. 2 and 3 demonstrate that a thirdharmonic wave can be generated with a linear polarized fundamental wavewhereas the third harmonic vanishes for a circularly polarizedfundamental wave.

It has been shown from the above, that an enhanced phasematched thirdhannonic neodymium laser light wave, utilizing anomalous dispersion, hasbeen generated by the use of the third harmonic generating cell 5. Thisthird harmonic signal is generated completely independent of thedirection of the laser beam propagating through the medium of cell andthere is little if any damage to the dye-solvent solution due to highpower laser radiation.

The laser-optical systems of FIGS. 2 and 3 have been used to illustratethat an absorption cell containing a solution consisting of the dyefuchsin and a solvent does generate a third harmonic signal from alinear polarized fundamental wave of a neodymium laser. Thus, the cellcontaining the dye in solution may be used in a system in which thethird harmonic wave of a neodymium laser may have practical application.In using the system, the light upon emerging from the cell is directedthrough any appropriate optical system to obtain results which may bederived from application of a third harmonic wave of a neodymium laseroutput. Thus, the appropriate optical system for making practical usesof the third harmonic wave will replace elements l6, l7, and 18 as shownin the system of FIG. 2. If desired, the light output of the neodymiumlaser may be directed directly into the absorption cell 5 therebyeliminating elements l4, l5, and 21. Therefore, a system for practicalapplication may include the neodymium laser, the third harmonicgenerator cell, and any additional elements needed in making use of thethird harmonic wave generated by the absorption cell 5. Since it is onlynecessary that the neodymium laser radiation pass through thephase-matched liquid, this liquid might be used or contained by meansother than the particular absorption cell 5, cited as an example.

Several applications employing a system including the third harmonicgeneration cell are the following:

( l Identification of third harmonic radiation.

(2) The system can be used as an optical switch or amplitude modulatorby varying the state of the fundamental laser polarization, a thirdharmonic intensity can be varied accordingly.

(3) Pulse narrowing can be accomplished with this device since the thirdharmonic pulse emerging from the cell is narrower than its fundamental.

(4) The device can be used in the measurement of pulsewidths,particularly ultrashort pulses such as obtained with mode-locked laserswhere a linear polarized laser beam is divided into two circularlypolarized components of opposite sense and they generate a thirdharmonic signal only when in' coincidence in passing through the thirdharmonic generating device. By varying the time delay between the twocircularly components the pulse widths can be determined by theirinterference.

The above examples are the most obvious uses of a system using the thirdharmonic generation cell; however, there are other applications whichmay be apparent to those skilled in the art.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. A system for generating a third harmonic wave from a linear polarizedfundamental wave;

said system comprising,

a laser source,

a radiation polarization means in optical alignment with said lasersource,

said polarization means generating linear polarized fundamental laserradiation,

said polarization means generating linear polarized fundamental laserradiation, a nonlinear liquid medium through which said polarized laserradiation propagates,

said nonlinear liquid medium including a solvent hexafluorisopropanoland an additive of dye fuchsin in said solvent,

said nonlinear liquid medium generating a third harmonic wave; and

means for positioning said nonlinear liquid medium in the path of saidpolarized radiation.

2. A system as claimed in claim 1 wherein,

said dye concentration is approximately 45 grams per liter of solvent.

3. A system for generating a third harmonic wave from a linear polarizedfundamental wave;

said system comprising,

a laser source,

a radiation polarization means in optical alignment with said lasersource,

a nonlinear liquid medium through which said polarized laser radiationpropagates,

said nonlinear liquid medium including a solvent hexafluoroacetonesesquihydrate, and an additive of dye fuchsin in said solvent,

said nonlinear liquid medium generating a third harmonic wave; and

means for positioning said nonlinear liquid medium in the path of saidpolarized radiation.

4. A system as claimed in claim 3 wherein,

said dye concentration is approximately 37.5 grams per liter of solvent.

1. A system for generating a third harmonic wave from a linear polarizedfundamental wave; said system comprising, a laser source, a radiationpolarization means in optical alignment with said laser source, saidpolarization means generating linear polarized fundamental laserradiation, said polarization means generating linear polarizedfundamental laser radiation, a nonlinear liquid medium through whichsaid polarized laser radiation propagates, said nonlinear liquid mediumincluding a solvent hexafluorisopropanol and an additive of dye fuchsinin said solvent, said nonlinear liquid medium generating a thirdharmonic wave; and means for positioning said nonlinear liquid medium inthe path of said polarized radiation.
 2. A system as claimed in claim 1wherein, said dye concentration is approximately 45 grams per liter ofsolvent.
 3. A system for generating a third harmonic wave from a linearpolarized fundamental wave; said system comprising, a laser source, aradiation polarization means in optical alignment with said lasersource, a nonlinear liquid medium through which said polarized laserradiation propagates, said nonlinear liquid medium including a solventhexafluoroacetone sesquihydrate, and an additive of dye fuchsin in saidsolvent, said nonlinear liquid medium generating a third harmonic wave;and means for positioning said nonlinear liquid medium in the path ofsaid polarized radiation.
 4. A system as claimed in claim 3 wherein,said dye concentration is approximately 37.5 grams per liter of solvent.